1. Field of Invention
This invention relates to marking systems in general and in particular to high frequency banding in marking systems.
2. Description of Related Art
The image quality defect known as banding is, within a printed image, a periodic modulation of lightness and darkness on a printed medium that runs in the marking process direction. Moreover, banding amplitude can change over time to produce different banding characteristics for printed images produced at different times. Banding and high frequency banding generally occur across the full width of an image, and may vary in amplitude in both time and in the direction perpendicular to the marking process direction, i.e., the cross-process direction. Banding and high frequency banding can be caused by a number of fluctuations that occur within the subsystems of a marking engine such as, for example, laser polygon Raster Optical Scanner (ROS) facet-to-facet reflectivity variation, intensity and spot size variation in a multibeam ROS, ROS polygon wobble, and photoreceptor velocity modulation.
A conventional approach to eliminate banding defects is generally to require the manufacture of parts/subsystems to tight tolerances. Alternative approaches include using active compensation schemes. For instance, a compensation scheme has been proposed wherein banding defects are being sensed with optical sensors in the developed image on the photoreceptor in real time, and then the development field is actuated according to a feedback control strategy in order to prevent the formation of the bands. In such an approach, accurate sensing of banding defects plays a critical role in the effectiveness of the compensation.
Generally, two types of sensors have been utilized to detect banding: toner area coverage sensors which are point sensors and array sensors. Toner area coverage sensors generally possess large apertures, and thus cannot resolve high frequency banding if the period is smaller than the size of the aperture. On the other hand, full-width array sensors typically provide high resolution in the cross-process direction. However, constraints on cost, limitations on the illuminator exposure intensity, and/or limitations on the size and processing of the collected image may require the sensor to have a relatively long integration time in the process direction. This long integration time generally prevents the detection of high frequency banding using these conventional data extraction techniques.
Generally, it is desirable to measure banding frequency, phase and/or amplitude using a single test pattern. Methods that are capable of identifying periodic patterns of unknown frequency and angle are generally well known in the field of halftone screening. An example includes screen finders for halftone screen identification. These methods utilize the moiré patterns caused by the interaction between two periodic functions one superimposed, scanned or rescreened. However, the design of moiré image patterns and the associated detection methods are generally not suitable for banding measurement within a marking device.